Updates sent to Amazon's Kindle tablets add new features to the devices and do not block them. As long as a Kindle device is connected to a wireless network, updates are downloaded and installed automatically.
If your Kindle does not receive an update on its own, you can also download it manually. To do so, download the update to your computer using the link on an update page, and transfer it to your Kindle via USB cable.
Some of the features you can add via updates are Amazon Underground, UI redesigns, typography improvements and password protection. Some updates also fix existing bugs.